1. Field of the Invention
The invention relates generally to the field of exploration and production of hydrocarbons from wellbores. More specifically, the present invention relates to an apparatus and method for estimating the temperature of connate fluid sampled from within a subterranean geological formation.
2. Description of Related Art
The sampling of fluids contained in subsurface earth formations provides a method of testing formation zones of possible interest by recovering a sample of any formation fluids present for later analysis in a laboratory environment while causing a minimum of damage to the tested formations. The formation sample is essentially a point test of the possible productivity of subsurface earth formations. Additionally, a continuous record of the control and sequence of events during the test is made at the surface. From this record, valuable formation pressure and permeability data as well as data determinative of fluid compressibility, density and relative viscosity can be obtained for formation reservoir analysis.
Early formation fluid sampling instruments, such as the one described in U.S. Pat. No. 2,674,313, were not fully successful in commercial service because they were limited to a single test on each trip into the borehole. Later instruments were suitable for multiple testing; however, the success of these testers depended to some extent on the characteristics of the particular formations to be tested. For example, where earth formations were unconsolidated, a different sampling apparatus was required than in the case of consolidated formations.
Down-hole multi-tester instruments have been developed with extensible sampling probes for engaging the borehole wall at the formation of interest for withdrawing fluid samples therefrom and measuring pressure. In downhole instruments of this nature it is typical to provide an internal draw-down piston which is reciprocated hydraulically or electrically to increase the internal volume of a fluid receiving chamber within the instrument after engaging the borehole wall. This action reduces the pressure at the instrument/formation interface causing fluid to flow from the formation into the fluid receiving chamber of the tool or sample tank. These pistons accomplish suction activity only while moving in one direction. On the return stroke the piston discharges the formation fluid sample through the same opening through which it was drawn and thus provides no pumping activity. Additionally, unidirectional piston pumping systems of this nature are capable of moving the fluid being pumped in only one direction and thus causes the sampling system to be relatively slow in operation.
The multi-tester instrument includes one or more internal pumps and associated control circuitry which permits the flexibility of selective “direct” pumping, where formation fluid is drawn from the formation and pumped directly into a sample tank and selective “indirect” pumping, where the pressure of an internal sample tank chamber is lowered, thus permitting filling of the sample chamber of the tank by formation fluid solely responsive to the influence of formation pressure. As the sample chamber is filled, a free piston within the sample tank will be moved by formation pressure until it comes into contact with an internal end wall or other internal stop of the sample tank.
After the sample tank has been withdrawn from the wellbore, along with the formation testing instrument, the pressure within the fluid supply passage from the instrument pump to the sample tank is maintained at the preestablished pressure level until a manually operable tank valve is closed. Thereafter, the pump supply line is vented to relieve pressure upstream of the closed sample tank valve. After this has been accomplished, the sample tank and its contents can be removed from the instrument body simply by unthreading a few hold-down bolts. The sample tank is thus free to be withdrawn from the instrument body and provided with protective end closures, thus rendering it to a condition that is suitable for shipping to an appropriate laboratory facility.
Sampling devices have been developed that include the functions and capabilities of measuring the temperature of the connate fluid flowing into the sampling device. The measured temperature may not be accurate because some amount of heat transfer will occur between the connate fluid that flows from the formation 6 and the sampling device. This heat transfer thereby alters the fluid temperature somewhat from its original value. The ambient conditions of the borehole 5 can also contribute to that temperature change. Accordingly the values of temperatures measured by temperature probes within the sampling device are not fully representative of the actual temperature of the connate fluid within the formation 6. Moreover, the above mentioned references concerning these sampling devices do not recognize the temperature gradient between the actual formation connate temperature and the sampled temperature, and thus do not provide an apparatus or method for obtaining the true temperature of the formation connate fluid.
Generally connate fluid samples collected in this fashion are used in a pressure volume temperature (PVT) analysis to determine the relative sample volume at various temperature and pressures. The raw PVT data may be used to tune a model (Equation of state EOS) that quantifies the gas and liquid phase at surface and pipe line pressure and temperature. EOS may then used to estimate the volume of produced hydrocarbon at gas and liquid state. The reservoir pressure and temperature are needed to tune the model at the reservoir condition.
As shown in FIG. 1, the sampling of subterranean formation fluid typically involves the insertion of a sampling tool 10 within a wellbore 5 that intersects the subterranean formation 6. Generally the tool 10 is inserted on the end of a wireline 8 or other armored cable, but can also be disposed within the wellbore 5 on tubing (not shown). When wireline 8 is used, it is typically maintained on a spool from which the tool 10 is reeled within the wellbore 5. When it is established that the tool 10 is adjacent to the region of the formation 6 where sampling is to occur, rotation of the spool is ceased thereby suspending the tool 10 at the proper depth within the wellbore 5. Upon suspending the tool 10 at the predetermined downhole depth, the urging means 12 is extended from the tool 10 that pushes the tool 10 against the inner diameter of the wellbore 5 on the side of the tool 10 opposite to the urging means 12. A probe 14 is provided on the tool 10 opposite to the urging means 12 such that activation of the urging means 14 causes the probe 14 to pierce the inner diameter or wall of the wellbore 5 and extend a small distance into the formation 6. The probe 14 has an annular configuration thereby allowing for fluid flow through its inner annulus. Within this annulus of the probe 14, subterranean fluid can flow from the formation 6 to within the tool 10 for storage and subsequent analysis.
Therefore, there exists a need for a connate fluid sampling system and method capable of sampling connate fluid, wherein the true formation temperature of the connate fluid can be found.